Aircraft using forward motion of the main wing or wings to achieve lift, must employ a horizontal stabilizer having pivotable control surfaces called "elevators" to control the nose-up or nose-down attitude of the aircraft, otherwise known as the pitching motion of the aircraft. This horizontal stabilizer can be mounted forward or aft of the main wing.
In most conventional aircraft today, the horizontal stabilizer is in the form of the tail assembly and is mounted aft of the main wing. In the canard design aircraft, the horizontal stabilizer in the form of a forward wing is mounted forward of the main wing. The forward wing is called a canard.
Canard design aircraft have been increasing in popularity in recent years as they are, in some respect, safer than conventional aircraft in that they are virtually stall and spin proof.
In conventional aircraft, it is essential that control of the aircraft by the aft tail mounted stabilizer be retained at all times, even though the main wing itself has ceased to provide lift and is in a stall condition. This control can only be achieved by ensuring that the main wing always stalls before the tail mounted stabilizer. In the case of a canard design aircraft, the opposite holds true, and the canard must stall and lose lift before the main wing. In both cases the effect is the same; in a stall, the nose of the aircraft pitches down, restoring lost air speed and regaining lift for the main wing. However, there is a fundamental difference between these two methods of stall recovery. In the case of the aft tail mounted stabilizer, the nose drops because the main wing has stalled. With a canard design aircraft, the canard stalls, causing the nose to drop, but the main wing never stalls. Therefore the canard design aircraft is considered to be far safer because stalls and resulting spins are virtually impossible to initiate. In addition, aside from the safety aspect, canard design aircraft provide the designer with a cleaner more aerodynamic shape to work with for better performance and economy due to the absence of a tail assembly.
One particular type of canard design aircraft has both a canard and an aft tail mounted stabilizer in an attempt to obtain the benefits of both conventional design and canard design aircraft (referred to as a three surface aircraft). The present invention relates primarily to a two surface canard design aircraft, although may be applied to the three surface aircraft.
The foregoing description of the capabilities of canard design aircraft and conventional aircraft apply to conditions during normal flight. In takeoff and landing situations, canard design aircraft are generally considered to have at least one disadvantage when compared to conventional aircraft. It is desirable that speeds for takeoff and landing be as low as possible to allow aircraft to use shorter runways for landing. In order to retain good high speed characteristics and still be able to obtain sufficient lift at low speeds for takeoff and landings, flaps were developed for conventional aircraft. Flaps are pivotally mounted surfaces attached to and forming part of the main wing at the trailing edge. Their purpose is to increase lift at low speeds for take off and landing. When the flaps are lowered, the additional effect is that the center of lift of the main wing moves aft causing a nose down pitching moment. With conventional aircraft, an aft tail mounted stabilizer is designed to retain sufficient authority to control the aircraft when the main wing is stalled, and thus is able to overcome nose down pitching which results from lowering the flaps. In canard design aircraft, the canard is designed to stall before the main wing, with the result that the canard generates lesser lift and is incapable of preventing nose down pitching of the aircraft when the center of lift moves aft with deployment of the flaps. Therefore it is generally accepted that a canard designed aircraft cannot be fitted with flaps. Thus take off and landing speeds for canard design aircraft are higher than most conventional aircraft fitted with flaps.